Erythropoietin is a glycoprotein hormone produced in the kidney that controls erythrocyte production.
Special cells send out erythropoietin in response to hypoxia, or a decreased red blood cell concentration. Erythropoietin stimulates bone marrow to produce more red blood cells, which leads to more oxygen out to your muscles. It can increase red blood cell production by 5-8 times. You can tell that production has increased because the blood will contain a lot of reticulocytes, which are slightly immature red blood cells.
Normal levels of erythropoietin in blood are 0 to 19 milliunits per milliliter. Erythropoietin concentration decreases with some bone marrow disorders, since the marrow won't respond by producing more blood cells like it's supposed to, or some kidney disease, since the erythropoietin-producing cells are damaged. Concentration increases in patients with polycythaemia rubra vera.